2016-06-04 Crystal Radio Coils, Crystal Diodes

I was reading some of Geo John’s projects and came across a crystal radio set that he built using two tuned coils. A crystal radio set refers to the type of detector used, which was once a galena crystal and a ‘catwhisker’ to find a sensitive spot on the crystal. Nowadays, that is no longer used, it has been replaced by germanium and silicon diodes.

The idea of the circuit was to get high selectivity by tuning the antenna input coil with a series capacitor, and also tuning the station selector coil with a parallel capacitor. These two coils are electrically linked together by a ‘link’ which consists of a few turns on one coil connected to the same number of turns on the other coil. The author inserted a low value resistor – 22 ohms – in series with one link wire to make the coils less tightly coupled. These two coils have to be tuned with two separate tuning capacitors, so the operator has to do one, then the other to get peak volume.

Both tuning capacitors were dual 335 pF with both sections in parallel for 670 pF total. For the coils, he used two T80-15 powdered iron cores with Litz wire, which is expensive and hard to obtain, but gives better results than regular enameled wire. He first used 30 AWG wire but wasn’t satisfied with its performance. He also showed a similar Heathkit crystal set that uses two tuning capacitors. This use of two tuning capacitors reduces interference from other powerful stations close by in frequency.

Interesting development

I was looking for Litz wire on eBay and found a seller with the type that he used in his project. But I found another thing on the seller’s store. I started reading about this in the links he included.

New Way Of Detecting For Crystal Sets

The author, Geo John, talked about using diodes for detecting the radio signal, but never mentioned this method. In the last few years, I and others found out about zero voltage MOSFETs, one made by ALD. We wanted to use these for Joule Thiefs, because they would work at much lower voltages than regular silicon transistors.

But shortly after these became available, the Crystal Set builders found out about them and started to use them as detectors in crystal sets. They give an audible signal with a very small or even no antenna. This is much more sensitive than the usual 1N34 germanium diode used in most crystal sets, which requires an antenna and ground for good performance. Some of the radio signal is fed through the MOSFET, while the MOSFET’s gate is connected to the tuning ‘tank’ circuit. Every time the radio frequency signal hits a peak voltage, it turns on the MOSFET, and it lets the higher current through to the earphone. So very low level signals that could not get through the germanium diode can get through the MOSFET to the earphone and be heard. Even though this MOSFET is more sophisticated and expensive than the diode, the ‘crystal set’ still is powered only by the signal of the radio station, so it is still just like a crystal set.

I would like to add that I have found out from experience that connecting any type of transistor to an antenna is a very bad idea. Eventually a thunder storm comes by and the transistor becomes a melted lump of silicon after the first lightning flash. The high voltages induced in the antenna will get through to the radio and take the path of least resistance, which is any semiconductor – crystal, transistor, whatever. If you are doing this, it is a good idea to put the semiconductor in a socket, and tape a small bag of spares inside of the radio. Some might say that a lightning arrestor will take care of the problem, but I have seen many transistors protect the lightning arrestor. If you want protection, install a ground at the antenna’s lead into the building, and connect the antenna wires to the ground when it is not being used.

I saw a lightning bolt strike a seven strand aluminum wire about 9mm or 3/8 inch diameter. It left about 7 meters or 25 feet of the wire melted into short lengths scattered on the ground. Mother Nature does NOT play around when it comes to lightning! :-O

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